The present invention relates to the preparation of a micaceous product preferably in the form of a mica tape comprising a support coated with a solvent-free resin, combined with a mica sheet.
More specifically, the present invention relates to a process for preparing a mica tape which is capable of being impregnated subsequent to the preparation process itself.
The present invention also relates to a novel micaceous product.
The electrical and optionally thermal insulation of copper components of electric engines, such as rods or coils, is achieved by taping up the said parts using micaceous products, more specifically micaceous tapes.
These micaceous tapes are in the form of a support impregnated with a resin, for example an epoxy resin, combined with a mica sheet. This mica sheet may be mica paper, optionally reinforced with a small amount of resin, for example epoxy resin.
Two main families of these tapes exist.
The first family comprises tapes known as xe2x80x9csaturated tapesxe2x80x9d or xe2x80x9cprepregsxe2x80x9d, which are tapes manufactured from mica paper that is highly impregnated with a resin of Novolac type and having a glass fabric as support. The resin content is usually between 30% and 40% relative to the total weight of the tape.
During manufacture, the epoxy resin is brought to stage B, i.e. it has already undergone curing. Next, this type of product may be taped around the part to be insulated and will then undergo a heat treatment of about from 160xc2x0 C. to 180xc2x0 C.
Document JP-07 149 928 discloses a product in the form of a saturated mica tape manufactured conventionally. It appears that a solvent-free epoxy resin is then used in the second phase of the manufacture, i.e. after taping the tape around the machine, in order to obtain adequate electrical insulation.
Document EP-A-0 735 071 discloses a solvent-free resin composition optionally intended for manufacturing a micaceous tape of saturated type, i.e. a tape which is already totally impregnated and consequently has a relatively high resin content.
Document GB-A-2 083 849 discloses a process for preparing insulating tapes, which consists in impregnating a mica support at room temperature using a solvent-free resin, followed by attachment to a support and heating of the assembly so as to reduce the viscosity of the resin, which will promote the deep impregnation of the mica paper. A tape of xe2x80x9cprepregxe2x80x9d type which is hence already highly impregnated will thus again be obtained.
The second family comprises so-called xe2x80x9cporousxe2x80x9d tapes, which have a relatively low resin content of between about 4% and 10% relative to the total weight of the tape. These tapes are capable of being impregnated subsequent to the process for manufacturing the said tapes and will be subjected, after taping up, to a xe2x80x9cVPI (vacuum pressure impregnation)xe2x80x9d treatment, which makes it possible in a second phase to carry out an impregnation with a resin which is usually solvent-free. The xe2x80x9cVPIxe2x80x9d treatment consists in immersing in the impregnation resin the copper parts taped up with the porous mica tape, while applying vacuum in order to remove any infiltrated air; next, the vacuum is broken and a certain pressure is applied for several hours in order to make the post-impregnation resin penetrate into the mica insulant. The prosecution of the entire post-impregnation process and the temperature of the resin are set by the nature of this resin, the thickness of the insulation to be impregnated and the porosity of the mica tape used.
Before taping up, very flexible, non-adhesive dry tapes are obtained, which are distinguished by truly exceptional absorption capacity. They are consequently used for high-voltage machines (of: up to 1000 MVA).
To prepare such tapes of the second family which are considered as capable of being impregnated, the xe2x80x9csprayingxe2x80x9d technique is known, which consists in spreading and impregnating a mica paper with an epoxy resin in solvent medium and then combining it with a support.
To prepare tapes capable of being impregnated, it is also known practice to use techniques of dusting a solid resin either on a mica sheet or directly onto the support, and then to attach the two components under pressure and heat. In particular, the publication EP-A-0 194 974 discloses a process for preparing thin mica tapes capable of being impregnated and comprising an incorporated accelerator, characterized in that a thin mica sheet is dusted with a powdered varnish free of hardener, next either the side of the thin mica sheet dusted with the varnish is bonded under pressure or heat with a glass fabric or a felt serving as support, or a glass fabric is impregnated with a liquid accelerator or a solution of a liquid or solid accelerator in a low-boiling solvent and the support thus obtained is bonded under pressure and heat with one side of the mica sheet dusted with varnish, and the laminate obtained is then impregnated with a liquid accelerator or a solution of a liquid or solid accelerator in a low-boiling solvent. Another possibility consists in impregnating a mica sheet with a liquid accelerator or a solution of a liquid or solid accelerator in a low-boiling solvent, in next dusting the impregnated mica sheet with a powdered varnish free of hardener, and then either in impregnating a glass fabric with a liquid accelerator or solution of a liquid or solid accelerator in a low-boiling solvent and bonding the support thus obtained under heat and pressure with the side of the thin mica sheet which has been dusted with powdered varnish, or in bonding the felt serving as support under pressure and heat with the side of the thin mica sheet which has been dusted with powdered varnish. However, it is observed that by using this process, the dusting operation with a varnish requires the use of a certain amount of varnish. In particular, the varnish will have a tendency, when it has been dusted onto the support, to pass through the mesh and be found on both faces of the support.
The present invention aims to provide a process for manufacturing micaceous products capable of; being impregnated, which requires the use of a low content of resin.
The present invention aims to provide a technique for avoiding the use of solvents for the preparation of micaceous products capable of being impregnated, such as mica tapes of porous type.
The present invention aims also to allow the preparation of such tapes which have increased flexibility while at the same time having sufficient or even increased adhesion properties.
In addition, the present invention aims to provide in the specific case of preparing roiled-up tapes, a process which avoids the problem of bonding between successive turns.
The present invention relates firstly to a process for preparing a micaceous product capable of being impregnated, which is preferably in the form of a mica tape obtained by combining a support and a mica sheet, characterized in that:
the support or the mica sheet is coated with a resin or an adhesive or alternatively a mixture of solvent-free resins, with the aid of coating cylinders at a working temperature for the coating operation,
the support is combined with the mica sheet, and they are subjected to a pressure and temperature treatment at a second working temperature.
The expression xe2x80x9cworking temperature of the coating operationxe2x80x9d should be understood as meaning the temperature to which the mixture of solvent-free resin is brought in order to be able to coat the support or the mica sheet. This temperature is usually between 40xc2x0 C. and 200xc2x0 C. This means that the resin should be in a non-solid form, i.e. in the more or less viscous or liquid form.
The expression xe2x80x9ca pressure and temperature treatmentxe2x80x9d means a treatment at a temperature of between 40xc2x0 C. and 200xc2x0 C. for a pressure of between 0 and 20 bar in order to allow attachment of the support to the mica sheet.
Among the possible examples of resin intended for coating which may be mentioned are solvent-free epoxy resins, solvent-free silicone adhesives or any other solvent-free resin which is of adequate viscosity at the working temperatures of the coating operation.
The support may be either a film or a fabric, or even a felt. The mica sheet is preferably a conventional mica paper manufactured according to the usual papermaking techniques. This mica paper may be a 100% mica paper or optionally a mica paper which has been reinforced beforehand with an impregnation resin, for example an epoxy resin, by, a conventional implementation process such as impregnation by coating optionally in solvent medium.
It may be envisaged to incorporate an accelerator either with the resin used for the Coating or during a prior step during the preparation of the mica sheet into the so-called impregnation resin, or alternatively directly onto the support or onto the mica sheet itself.
In the first embodiments, the accelerator is mixed directly with the resin.
In general, it may be envisaged to use an accelerator which is in the form of a nitrogen compound such as an amine or an organometallic compound such as zinc naphthanate, or any other compound having the desired catalytic effect.
A second object of the present invention relates to a micaceous product capable of being impregnated, which is preferably in the form of a so-called porous mica tape and which comprises, on the one hand, a support impregnated by solvent-free coating with a resin and a mica sheet, or alternatively a mica sheet impregnated by solvent-free coating with a resin and the support itself, these two components: being attached to each other.
It is observed that when the support is a fabric, there will be deposits of resin essentially on the intersections of the yarns of the weft with those of the warp. More specifically, traces of resin will be observed exclusively at the intersections where the weft yarn superimposes the warp yarn. This makes it possible to obtain, in a particularly advantageous manner, very precise control of the resin content on the finished tape, this resin content being very much lower than that for the products obtained by the processes of the prior art (spraying or use of a solid powder). This also makes it possible to maintain or even enhance the flexibility properties and to limit the risks of bonding between turns when the tape is rolled up.
The fabric will preferably be a fabric or a silk made of woven glass fibres, having a weight per unit area of from 20 g/m2 to 50 g/m2. The mica sheet, and preferably the mica paper, has a weight per unit area of between 100 g/m2 and 250 g/m2.